Homozygosity for a single mutation (HFE C282Y) is responsible for the vast majority of cases of hemochromatosis, yet phenotypic expression in homozygotes varies widely. The hypothesis to be tested in this application is that modifier genes are responsible for the phenotypic variability. To test this hypothesis, we propose two specific aims. Specific Aim 1: Utilize murine chromosome substitution strains (CSSs) to identify chromosomes that influence the iron phenotype. Iron metabolism in inbred mice varies in a strain-specific manner. We have established that mice of the A/J strain have a "high iron" phenotype. They absorb twice the amount of iron than C57BL/6J mice absorb and also deposit more newly absorbed iron in the liver, have higher transferrin saturation, higher liver iron content and transfer more iron from macrophages to hepatocytes. The CSSs we will utilize are on a C57BL/6J background, but each strain is homozygous for one chromosome (1-19, X,Y) donated from A/J. A CSS strain in which the iron phenotype differs from C57BL/6J will immediately identify a chromosome containing a modifier gene(s). CSSs of interest will be crossed with Hfe mutant C57BL/6J mice to establish that the chromosome identified modifies the Hfe phenotype. Specific Aim 2: Identify candidate modifier genes by narrowing the regions of interest on the chromosomes identified in Specific Aim1. Recombinant congenic strains (RCS) have been generated from the C57BL/6J an A/J strains. RCS containing segments of chromosomes identified in Specific Aim 1 will be phenotyped to narrow the candidate modifier region. Genes localized to the candidate region will be considered candidates for genes that modify the iron phenotype and hence affect expression of hemochromatosis.